Solid State Relay

[ptonsparky]

A customer has piece of equipment that had a VFD installed a few years ago replacing one of the motor starters. All 480v control. They are using the 250v 3A rated VFD ouptput relay as an interlock for the other motor starters. I would like to connect a solid state relay to the output of the VFDs electronic relay. At $60 + shipping and minimal wiring changes I don't see a down side. Yet. Am I missing something?

 

[Jraef]

A customer has piece of equipment that had a VFD installed a few years ago replacing one of the motor starters. All 480v control. They are using the 250v 3A rated VFD ouptput relay as an interlock for the other motor starters. I would like to connect a solid state relay to the output of the VFDs electronic relay. At $60 + shipping and minimal wiring changes I don't see a down side. Yet. Am I missing something?

As an interposing relay to isolate the other starter coils from damaging the VFD output? Good idea but why an SSR? Some cheap SSRs won't like AC coil inrush, but at $60 sounds like you are not choosing a cheap one. Probably fine, but future troubleshooting may be problematic because SSRs "leak". So if you are looking to see if the SSR is on or off with a DVM you may not be able to tell. I would just use a regular relay or if you want higher current, one of those mini-contactors. they are a lot cheaper than that too.

 

[ptonsparky]

The low end VFD has a 50ma limitation on its control power, as far as I can tell from the installation manual.

SSR was about the only thing I could think of that would operate on such a low current. The one I have in mind is rated @ 10 amps.

 

[Jraef]

The low end VFD has a 50ma limitation on its control power, as far as I can tell from the installation manual.

SSR was about the only thing I could think of that would operate on such a low current. The one I have in mind is rated @ 10 amps.

Wow I see the problem. 50ma is like nothing! It's probably intended for 24VDC. Did you check to see if the VFD provides a limited amount of 24VDC for this purpose? Many do, even the low end versions. It would have a limited capacity as well, but is often the same as the rating of the SSO. In other words you tap off of the +24VDC, run it through the little SSO on the VFD and switch a little ice cube relay with a 24VDC coil as an interposing relay. You can get one with 15A contacts that only draws 0.7W on the coil, you have 1.2W capacity at 24VDC on that SSO (but the extra capacity of the built-in supply may be lower).

 

[weressl]

A customer has piece of equipment that had a VFD installed a few years ago replacing one of the motor starters. All 480v control. They are using the 250v 3A rated VFD ouptput relay as an interlock for the other motor starters. I would like to connect a solid state relay to the output of the VFDs electronic relay. At $60 + shipping and minimal wiring changes I don't see a down side. Yet. Am I missing something?

The SCR load on an ASD output is a sure way to drive the ASD 'nuts', if I undetsyand the question correctly.

 

[ptonsparky]

The Form A contacts on the drive are rated at 250V, all the coil voltages for other starters etc that must interlock with the VFD are 480v.

A digital output is available to operate a "coil". You may be right about it not wanting to operate an SCR. I'll use 12v via the Form A contacts to operate the SSR.

From installation manual:

This output circuit is a currentsourcing​

type rated at 12 VDC and 50 mA maximum.